It has been found quite important, since the advent of air transportation in cold climate, to evaluate and control icing on the surface of a flying vehicle. It is essential to be able to determine not only the presence and thickness of an ice layer; it is also important to be able to distinguish, from a distance, between ice, de-icing liquid, snow, and water presence on the surface in question.
Among known techniques for ice thickness measurement, a distinction can be made between remote and non-remote measurements. Electrical, acoustic, mechanical and optical devices have been proposed to date for this purpose.
In French Patent No. 2618543 issued to Clerc, a surface analyzer for a motor vehicle is described as having electromagnetic emitters whose signals are reflected from road surface for processing by means of receivers and a processing unit so as to determine the state of the road surface.
European Patent Application No. 461,953 proposes another device for examining road surface, including a laser that emits a light beam towards the surface. The reflected light is collected by a detector which generates signals analyzed by a microprocessor. The device is attached to a vehicle. The source is controlled by a circuit so that the beam spot on the road surface is immobile relative to the surface for a certain length of time. The device is useful to differentiate between ice, water and snow.
A device based on the degree of absorption of infrared radiation for detection and thickness measurement of ice is described in U.S. Pat. No. 4,808,824. The reflective and absorptive qualities of ice in the infrared region can also be used to detect ice and measure the thickness according to DE Patent 4008280.
Other optical/electrical methods and devices for ice evaluation on the road surface or other surfaces are described in U.S. Pat. No. 5,218,206 (Schmitt et al.), EP application No. 405,625 (Federow), U.S. Pat. No. 5,014,042 (Michoud) and U.S. Pat. No. 4,690,553 to Fumizu et al.
There is still a need for a simple and reliable method and device for remote, non-contact detection and thickness measurement of ice. At the same time, it is recognized that a layer of a partly transparent solid other than ice, e.g. a polymer, on a smooth solid surface, has certain characteristics similar to those of ice, and it may be necessary to detect or measure the thickness of, such layer.